This invention relates to field electron emission materials, and devices using such materials.
There have been many proposals for broad-area field electron mission materials, many or most of which concentrate on the use of diamond or amorphous carbon as an emitting material of special significance. In the context of this definition, a broad-area field emitter is any material that by virtue of its composition, micro-structure, work function or other property emits useable electronic currents at macroscopic electrical fields that might be reasonably generated at a planar or near-planar surface.
The reader is referred to UK Patent 2 304 989 (Tuck, Taylor and Latham) for examples of emitting materials, including many other than diamond. The present application relates particularly to field electron emission materials involving a primary interface region between a conductive surface, or an electrically conductive particle on it, and an insulating layer, and a secondary interface region between that insulating layer and the environment in which the field electron emission material is disposed.
A critical issue in insulator-based field emitting systems is the injection of electrons from a substrate (often a metal) into the conduction band of the insulator.
FIG. 1a is a reasonable representation of the current state of knowledge of such systems, although this still falls short of an exact description. In particular the sharp cut off in the density of states at the band edges is unlikely in highly heterogeneous amorphous materials. However, with these caveats in mind, such a diagram is a useful representation. Electron emission through a dielectric coating is effectively controlled by three factors: injection of the electrons 1503 into the dielectric from the conducting substrate 1500; transport through the dielectric to the surface as indicated by line 1511; and subsequent escape through or over the surface barrier 1506 into the vacuum 1502. A practical insulating layer will have both donor 1507 and acceptor defect sites 1509 in the band gap. The most notable effect is when there are donor states in the band gap relatively close to the bottom of the conduction band. In this case electrons from the donor states 1507 tunnel back into the metal and a Schottky barrier 1510 is formed, see also FIG. 1(b), which enables electrons to tunnel through it from the metal into the conduction band. Bayliss and Latham (K. H. Bayliss and R. V. Latham, Proc. Roy. Soc. Lond. A 403 (1986) 285-311) have described the conditions required for forming such a Schottky barrier and its significance to electron emission into the dielectric. The Schottky barrier has an associated forward voltage drop. This becomes a particular issue as the particle size is reduced in the metal-insulator-metal-insulator-vacuum (MIMIV) emitters described by Tuck, Taylor and Latham (UK Patent 2304989) to enable them to be used in gated structures such as those described in our patent application GB 2 330 687. Whilst the electric field across the MIM region of a MIMIV emitter can be maintained by reducing the insulator thickness, the absolute voltage will fall to values below the forward voltage drop of the Schottky barrier thus stopping injection of electrons into the insulator.
A more general discussion of the metal-insulator contact in the case of diamond and diamond-like carbon is given by Robertson (J. Robertson, Mat. Res. Soc. Symp. Proc. 471 (1997) 217-229).
Transport through the dielectric depends critically on its nature. For relatively defect-free material, transport will be in the conduction band, with lattice scattering limiting conduction. Electrons may become ballistic rather than staying close to the bottom of the conduction band (D. J. DiMaria and M. V. Fischetti, Excess electrons in dielectric media, eds Ferradini and Jay-Gerin, p315-348, (CRC Princetoun:1991) ISBN 0849369622). By contrast, in a glassy material, with many donor and trapping sites, conduction will be dominated by the Poole-Frenkel effect, field-assisted ionisation of donors and traps, and the electrons will remain close to the Fermi level. In general conduction is non-ohmic with evidence of saturation effects, presumably due to space charge in some cases.
The final step is the emission of electrons from the dielectric surface into vacuum. In the case of hydrogen terminated diamond which has a negative electron affinity, and with the electron transport in the conduction band, there is no barrier to overcome and all electrons arriving at the surface will be emitted. In the case of a low positive electron affinity, such as an un-terminated diamond surface, there is usually sufficient electron heating in the transport to the surface to allow emission through thermionic and thermally enhanced tunnelling. For higher electron affinities, either the field at the surface must be high enough to enable tunnelling or there must be sufficient ballistic electrons that can pass over the barrier. Otherwise the surface must be modified to lower the effective electron affinity. Two possible means of achieving this lowering of the surface barrier are either modifying the surface composition e.g. by caesiating the surface or emptying surface donor states to leave a positively charged surface. The latter is the basis of the forming mechanism proposed by Bayliss and Latham.
An emitter of this type has initially to undergo a forming process. A relatively high switch-on field has to be applied to the device to obtain emission, but after removing this field, a much lower threshold field is required for emission. The actual mechanisms responsible for this behaviour are very difficult to establish because of the small dimensions of the conducting channels. Dearnaley et al. (G. Dearnaley, A. M. Stoneham and D. V. Morgan, Rep. Prog. Phys., 33, (1970) 1129-1191) suggest the formation of conducting filaments in the films for MIM (metal-insulator-metal) structures, while Bayliss and Latham suggest that a positive space charge is established in the insulator and at its surface.
Many papers on diamond and diamond-like-carbon field emitters make no mention of any forming process. However, a forming process is described for diamond emitters both by Xu et al. (N. S. Xu, Y. Tzeng, and R. V. Latham, J. Phys. D 26 (1993) 1776-1780) and by Givargizov et al. (E. I. Givargizov, V. V. Zhirnov, A. V. Kuznetsov and P. S. Plekhanov, J. Vac. Sci. Technol, B 14 (1996) 2030-31). It seems probable that other workers in this area concentrate on the reversible I-V characteristics of the emitters and may overlook the initial forming process.
It is probable that no one mechanism is appropriate to all situations and that a combination may apply in many cases.
For diamond films, the limiting factor to emission has been found by many workers to be the metal-diamond back contact (e.g. M. W. Geis, J. C. Twichell and T. M. Lyszczarz, J. Vac. Sci. Technol. B 14, (1996) 2060-67) and U.S. Pat. No. 5,713,775. However, no systematic method of overcoming this problem has been described.
Examples of ad hoc solutions are as follows.
Geis et al. showed that emission thresholds could be greatly reduced by introducing nitrogen into the diamond. The nitrogen defects are close enough to the conduction band to allow a Schottky barrier to be formed, reducing the field necessary to inject electrons into the diamond conduction band. Geis et al. considered also that xe2x80x9crougheningxe2x80x9d of the surfaces between metal and diamond was of considerable importance, roughening being of the order of 10 nm.
In fact it is likely that many examples of diamond and carbon-based films have an interface roughness of this order without intentional treatments. What is really needed is a more general strategy that can be applied to interfaces whether they are rough or smooth.
Schlesser et al reported improved emission for an annealed molybdenum-diamond interface (R. Schlesser, M. T. McClure, W. B. Choi, J. J. Hren and Z. Sitar, Appl. Phys Lett. 70 (1997) 1596-98)
Chuang et al reported improved emission for diamond deposited onto an annealed gold layer on silicon (F. Y. Chuang, C. Y. Sun, H. F. Cheng and I. N. Lin, Appl. Phys. Lett. 70 (1997) 2111-3).
In the last two cases it is probable that the Schottky barrier has been reduced or eliminated through the formation of some form of an ohmic contact. It is however difficult to be certain of the operating mechanisms of the recipes described in these publications as insufficient information is given about the nature of the diamond films.
Two more brief and general disclosures of emission from diamond films are C. Kimura, K. Kuriyama, S. Koizumi, M. Kamo and T. Sagino, Paper L-2, and T. Yamada, A. Sawabe, K. Okano, S. Koizumi and J. Itoh, Paper P-45, both papers being from IVESC ""98xe2x80x94The International Vacuum Electron Sources Conference held in Tskuba City, Japan. The first of these papers discusses the use of titanium and gold with phosphorus-doped diamond films, and notes the effect of different resistivities of the diamond film. The second of these papers discusses the use of both titanium and gold with nitrogen-doped and boron-doped diamond emitters. Both papers emphasise the perceived importance of diamond as a choice of emitter material to achieve good emission characteristics, but disclose no general teaching as to how to achieve good emission characteristics from materials generally.
Preferred embodiments of this invention aim to provide a systematic method for producing optimised low manufacturing cost field emitter materials based upon insulating coatings that have both a low emission threshold field and a controlled saturation above a chosen current density.
According to one aspect of the present invention, there is provided a method of creating a field electron emission material, comprising the steps of:
providing a substrate having an electrically conductive surface;
providing a plurality of electron emission sites on said conductive surface, each of said sites including a respective layer of electrically insulating material to define a primary interface region between said conductive surface, or an electrically conductive particle on it, and said insulating layer, and a secondary interface region between said insulating layer and the environment in which the field electron emission material is disposed; and
treating or creating the primary interface region of each said layer so as to enhance the probability of electron injection from said conductive surface into said layer, such treatment or creation comprising:
depositing a layer of material between said conductive surface and insulating layer, which layer of material has properties intermediate those of said conductive surface and said insulating layer; or
doping said conductive surface and/or insulating layer with a material that segregates out at said primary interface region during subsequent processing; or
reaction of the materials of said conductive surface and insulating layer; or
creating said primary interface region as a region of high electrically active doping, high defect density or intermediate chemical composition:
such that said primary interface region after said treatment or creation is either an insulator or graded from conducting adjacent said conductive surface to insulating adjacent said insulating layer.
Said layer of material between said conductive surface and insulating layer may be created by a gradual change in stoichiometry, composition or doping of the material of the layer, to reduce discontinuity.
A method as above may further comprise the step of selecting the properties of said insulating layer of each said site between its respective primary and secondary interface regions to limit the emission current flowing through said layer to a predetermined value.
Preferably, said primary interface region is a layer of material of low work function.
Preferably, said primary interface region is created as a region of high doping, defect density or intermediate composition.
Such a region of high defect density may be created by heat treating a major portion of a highly defective insulator material to create said insulating layer, whilst avoiding heat treatment of an end portion of said highly defective insulator material, which end portion then remains as said region of high defect density.
Preferably, said secondary interface region is provided by modifying the surface of said insulating layer, to enhance the probability of electron transmission from said insulating layer to said environment.
Modification of said surface may be by a local increase in defect density of the material of the insulating layer.
Modification of said surface may be by a gradual change in stoichiometry, composition or doping to reduce discontinuity.
Modification of said surface may be by local heat treatment of said insulating layer.
Said electron emission sites may be defined by tips or projections created on said conductive surface.
Said electron emission sites may be defined by electrically conductive particles coated on said conductive surface.
Said secondary interface region may be defined at a region of said insulating layer between a respective said particle and said conductive surface.
Said secondary interface region may be defined at a region of said insulating layer which is provided on a portion of a respective said particle which faces away from said conductive surface.
Each said particle may have a first layer of electrically insulating material between said substrate and particle and a second layer of electrically insulating material between said particle and environment, the arrangement being such that, in use, electron emission takes place by injection of electrons through one said primary interface region defined between said substrate and said first insulating layer, by injection of electrons through another said primary interface region defined between said particle and said second insulating layer, and by transmission of electrons through said secondary interface region defined between said second insulating layer and said environment.
Preferably, said first and second insulating layers are provided by respective portions of a common electrically insulating material.
Said insulating layer may be of a material other than diamond.
Preferably, the distribution of said sites over the field electron emission material is random.
Said sites may be distributed over the field electron emission material at an average density of at least 102 cmxe2x88x922.
Said sites may be distributed over the field electron emission material at an average density of at least 103 cmxe2x88x922, 104 cmxe2x88x922 or 105 cmxe2x88x922.
Preferably, the distribution of said sites over the field electron emission material is substantially uniform.
The distribution of said sites over the field electron emission material may have a uniformity such that the density of said sites in any circular area of 1 mm diameter does not vary by more than 20% from the average density of distribution of sites for all of the field electron emission material.
Preferably, the distribution of said sites over the field electron mission material when using a circular measurement area of 1 mm in diameter is substantially Binomial or Poisson.
The distribution of said sites over the field electron emission material may have a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 4 xcexcm diameter.
The distribution of said sites over the field electron emission material may have a uniformity such that there is at least a 50% probability of at least one emitting site being located in any circular area of 10 xcexcm diameter.
The invention extends to a field electron emission material produced by any of the above methods.
According to a further aspect of the present invention, there is provided a field electron emission device comprising a field electron emission material as above, and means for subjecting said material to an electric field in order to cause said material to emit electrons.
It will be appreciated that the electrical terms xe2x80x9cconductingxe2x80x9d and xe2x80x9cinsulatingxe2x80x9d can be relative, depending upon the basis of their measurement. Semiconductors have useful conducting properties and, indeed, may be used in the present invention as said conductive surface or particles. In the context of this specification, the or each said conductive surface or particle has an electrical conductivity at least 102 times (and preferably at least 103 or 104 times) that of said electrically insulating material.